I don't know which files you need to install as a BARE minimum because most of the X related packages contain an unhelpful generic package description. However, all of the files are located in the /x tree. All X related applications are located in the /xap tree. I would recommend just installing everything in the both trees. Later you can delete (removepkg or pkgtools) any unwanted X apps.
One of my old boxes has a very temperamental CD drive. A simple soft boot will cause the drive to stop working properly and even the BIOS will not find the drive. Because of that issue I developed a way for me to install or update Slackware using the original ISO image:
Installing Slackware Without a CD Drive
I hope this helps.